White light emitting LEDs is an efficient and harmless lighting technology with many advantages such as energy saving, environmental protection and long life, in which the white light is obtained by combining the blue light LED chips and yellow phosphor.
As a new lighting technology white light emitting LEDs work based on the following principles: A portion of blue lights emitted by blue light LED chips irradiate the phosphors, and after excited the phosphors re-emit yellow lights with a longer wavelength, or the mixture of green lights and red lights, or the mixture of yellow lights and red lights. The lights with longer wavelength re-emitted by phosphors combine with the residual blue lights emitted by blue light LED chips to provide light which appears to the eye as being nearly white in color. In traditional LED chip-package technology, the mixture of phosphor material and silica gel or resin is blended and solidified on the surface of LED chip. The traditional LED chip-package technology shows some inherent shortcomings: the difficulty of dissipating heat during lighting results in the rising of the working temperature of LED chip, leading to a shift in the wavelength of the emitted lights by phosphors, and also leading to a degradation of photo-luminescent intensity. In addition, because phosphors are set close to the surface of LED chips, the part of the lights emitted by the blue light LED chip are re-absorbed by the blue light LED chip, resulting in the loss of lighting efficiency. As taught, for example, in CN200910036503 ‘A white light emitting LED’ (“white LED”), white LEDs include blue light InGaN chip covered with Y-garnet phosphor re-emitting green-yellow lights after excited by a portion of blue lights emitted by blue light LED chip, and the green-yellow lights combine with the residual blue light from blue light LED chip to provide white light. As taught in CN200710079928, the white light emitting device comprises blue light LED chip and the mixture of orange phosphor and green phosphor which is solidified on the surface of blue light LED chip.